As semiconductor technologies develop, semiconductor devices are developed toward a direction with a higher device density and a higher integration level. A transistor is one of the most fundamental devices and is widely used. With developments of the semiconductor technologies, a length of a gate in a planar transistor decreases. Correspondingly, a controlling ability of a conventional planar transistor on channel currents becomes weaker, causing short channel effects and serious leakage current problems. The semiconductor device has a poor performance.
To better alleviate the short channel effects and suppress the leakage currents, fin field effect transistors (FinFETs) are widely used. A FinFET is a multi-gate device, and usually includes: fins on a semiconductor substrate; a dielectric layer on the semiconductor substrate covering a portion of sidewalls of the fins and with a top surface lower than top surfaces of the fins; gate structures on the top surface of the dielectric layer, on tops of the fins, and on the sidewalls of the fins; and sources and drains in the fins on two sides of the gate structures.
However, as the device density increases and critical dimensions decrease, it is more difficulty for forming the FinFETs, and the formed FinFETs have poor performances and low reliabilities.